1. Field of the Invention
The invention relates to a technical field of a highly-stabilized crystal unit by resistance welding or cold welding, and in particular, to a crystal unit whose unit base is formed of ceramic.
2. Description of the Related Art
A highly-stabilized crystal unit is generally used as a crystal unit of a constant-temperature type using a heater, and is free of the influence of an environmental temperature. Therefore, the crystal unit is capable of precisely maintaining frequency stability (for example, 10 ppb or less) thereof, and is incorporated in an oscillator for a base station among communication devices. As one of these crystal units, there has been proposed a crystal unit whose crystal element is horizontally held with respect to a metal base thereof (see, for example, JP-A-2005-348082).
FIGS. 3A to 3C are diagrams for explanation of one example of a related art crystal unit. FIG. 3A is a plan view of the related art crystal unit except for a cover thereof, FIG. 3B is a cross-sectional view taken along the line O-A in FIG. 3A, and FIG. 3C is an enlarged cross-sectional view of a part shown by O-shaped dotted line in FIG. 3B.
The related art crystal unit 200 is configured such that a crystal element 1 formed into, for example, a circular disk shape is held on a metal base 2, and is covered with a metal cover 3 so as to hermetically encapsulate the crystal element 1. The crystal element 1 is formed into a circular disk shape as an AT-cut crystal element, and has excitation electrodes 4 (4a and 4b) on the both principal surfaces thereof, and leading electrodes 5 (5a and 5b) are extended so as to include end faces at two places of the outer circumferential part at intervals at an angle of 120 degrees. The AT-cut crystal element has the principal surfaces which are planar surfaces formed by the X-axis and the Z′-axis of the axes (XY′Z′) newly formed by a rotation centering on the X-axis of the crystal axes (XYZ).
The metal base 2 has a metal flange 2b going around the outer circumference of a base main body 2a. The base main body 2a is formed of kovar consisting primarily of Fe so as to contain Ni and Co, and has lead wires 7 (so-called hermetic terminals) which are three in total, and are evenly arrayed at intervals at every angle of 120 degrees so as to be insulated to penetrate through the metal base 2 by glasses 6. The lead wire 7 has a nail-headed part 7a, that projects from the principal surface of the base main body 2a. Then, the glass 6 climbs the lead wire 7 through the both principal surfaces. A horizontal part of an L-shaped supporter 8 having a shoulder is connected to the nail-headed part 7a by spot welding. The metal flange 2b is formed into a crank shape in a cross-sectional view, and a horizontal part at one side thereof is welded onto the outer circumference of the base main body 2a. 
Then, the principal surface of the crystal element 1 is horizontally kept with respect to the metal base 2, and the three places of the two outer circumferential parts of the leading electrodes 5 (5a and 5b) which are extended and the outer circumferential part separated away from those at 120 degrees are held to the shoulders of the supporters 8 by electrically conductive adhesive (not shown). Then, the metal cover 3 formed into a concave shape of kovar and having a flange is connected to the other horizontal part of the metal flange 2b of the metal base 2 by cold welding or resistance welding.
The related art crystal unit 200 is mounted on a set substrate along with circuit elements composing an oscillator circuit and other elements. Then, the related art crystal unit 200 is mainly maintained at a constant temperature by a heater structure (not shown), to make a vibrational frequency stable to be, for example, 1 ppb (parts per billion) or less. Then, the set substrate is housed in a cassette to be set into a communication facility for a base station.
However, in the crystal unit having the above-described configuration, the metal base 2 has the base main body 2a formed of kovar, and the lead wires 7 whose nail-headed parts 7a project from the one principal surface. The lead wires 7 project from the one principal surface in order to ensure airtightness by the glasses 6, where the nail-headed parts 7a to which the supporters 8 are connected project. Thereby, the height of the base main body 2a of the metal base 2 basically is made higher by a projected vertical length of the lead wires 7, which causes the problem that the crystal unit is made higher. Incidentally, the thickness of the base main body 2a is required to be 1.3 mm in order to ensure airtightness by the glasses 6, and the projected length of the nail-headed parts 7a is set to 0.2 mm, and thus a total height of the base main body 2a and the nail-headed parts 7a becomes 1.5 mm.